1. Field of the Invention
The present invention relates to high-frequency semiconductor devices. The present invention more particularly relates to a high-frequency semiconductor device for use in a millimeter-wave or quasi-millimeter-wave circuit module for amplification, oscillation, or modulation.
2. Description of the Related Art
FIG. 1 is a plan view showing a structure of a high-frequency semiconductor device (HEMT) 1 that is currently in common use. In this semiconductor device 1, on the upper surface of a compound semiconductor substrate 2 (width: 300 .mu.m, length: 400 .mu.m) in which two active areas 3a and 3b are formed, a drain pad section 4 is disposed at one side against the line connecting the active areas 3a and 3b, which serves as a boundary, two gate pad sections 8 are disposed at the other side, and a source pad section 6 is disposed around the gate pad sections 8. Drain electrodes 5 extend from the drain pad section 4 toward the active areas 3a and 3b, respectively. Source electrodes 7, each divided into two portions, extend from the source pad section 6 to the active areas 3a and 3b, respectively. Gate electrodes 9 extend from the gate pad sections 8 toward the active areas 3a and 3b such that the gate electrodes pass between the two divided portions of the source electrodes 7. Each gate electrode 9 (which is very narrow and has a gate length Lg of 0.15 .mu.m) is disposed between the drain electrode 5 and the source electrode 7 in each of the active areas 3a and 3b.
When this semiconductor device 1 is mounted to an external circuit board, the source electrodes 7 are connected to the ground of the circuit board with bonding wire connected to the source pad section 6, and the gate electrodes 9 and the drain electrodes 5 are connected to RF signal lines of the circuit board with bonding wire connected to the gate pad sections 8 and the drain pad section 4, respectively. An RF signal is input to the gate electrodes 9 from the circuit board through the bonding wire and output from the drain electrodes 5 to the circuit board through the bonding wire.
When an RF signal is transmitted through a bonding wire, it is known that the RF characteristics of the semiconductor device 1 deteriorate due to the parasitic inductance of the bonding wire at high frequencies (especially at the frequency of a millimeter wave). As understood from the structure of the semiconductor device 1 shown in FIG. 1, since the source electrodes 7 are disposed very close to the gate electrodes 9, parasitic capacitance exists between the gate electrodes 9 and the source electrodes 7. Due to parasitic capacitance between the gate electrodes 9 and the rear surface of the semiconductor substrate 2 in addition to the above parasitic capacitance, the high-frequency characteristics of the semiconductor device 1 deteriorate.
To reduce parasitic components generated in a semiconductor device, flip-chip mounting using solder bumps may be employed instead of bonding wire. When flip-chip mounting is used, although parasitic inductance caused by bonding wire decreases, there is an increase in parasitic capacitance between an electrode pad and a circuit board. This method therefore does not essentially solve the problem.
Since the drain pad section 4 is spaced away from the source pad section 6 at the drain electrodes 5, parasitic capacitance between those pad sections 4 and 6 is small. Since the surface of the semiconductor is not covered by an electrode between the drain pad section 4 and the source pad section 6, however, a surface wave may be generated between those two electrodes, which causes a loss and deterioration of the RF characteristics.
As described above, a high-frequency semiconductor device having the conventional structure has a problem of characteristics deterioration caused by parasitic components, such as parasitic capacitance and inductance, and thereby yield in manufacturing semiconductor devices is reduced. It is very difficult to suppress the characteristics deterioration in a high-frequency semiconductor device having the conventional structure.